As a result of broadened optical fiber communication, low-priced communication of mass digital information is enabled. New service that applies mass digital information further promotes broadened communication, and the traffic of the Internet grows at a high rate, approximately doubling in two years. An optical fiber network where mass data is communicated at high speed over a relatively long distance of several km, with a basic trunk, a metropolitan area network, and an access line cover, has been developed. Optical link technology that uses optical linking is also effective in extremely short distances between information communication (ICT) devices, such as a server in a data center (several meters to several hundreds of meters), or in the information communication device (several to several tens of centimeters).
For network topology with plural nodes connected via an optical fiber, a transmission network has been configured in a complex combination of a linear chain network, a mesh network, a ring network, a multi-ring network, and a standby line access network, as well as in a point-to-point network, which is the most simple. In such complex transmission networks, it is estimated that damage when failure occurs in a transmission line because of disaster and disconnection is inscrutable. Now, therefore, a network configured to maintain required communication in a sudden change of communication environment, such as the suspension of a network in a disaster, and others, is demanded. A mesh network is a representative example of a network in which plural transmission paths exist based upon the abovementioned background. In a mesh network, when an active signal line is unavailable, switching to an alternative route is performed, and, at the time of disaster, a secure transmission path is also enabled.
The mesh network is configured so that a signal input to each node is output in at least three directions to secure an alternative route, and the scale of a matrix in an optical switch for switching paths increases. In an optical switch in which the scale of the matrix increases as described above, loss increases because the number of interfering paths for switching paths increases. The number of cross points between waveguides also increases in the case of a waveguide type. When such an optical switch is a mechanical type, an operational range widens, and loss increases by the quantity of an operational error as the scale of the matrix increases. In the meantime, in the optical switch in which the scale of the matrix increases, as the number of ports increases, the number of wires increases. The reason is that plural wires are prepared every time a network is increased, and as an installation cost greatly increases, the optical switch has poor scalability and a huge cost is required for the restoration of a transmission network. Moreover, when the alternative route is long, transit in the optical switch is frequent and loss applied to a transmitted signal by the optical switch also increases.
For a transmission network in which the securement of plural paths between different nodes is enabled, a transmission network configured by plural rings, each of which is acquired by connecting ends of the mesh network, is well-known (for example, refer to International Publication WO 2007/018164).